A high affinity, saturable and sterospecific (3H-)imipramine binding to crude synaptic membranes preparations from hippocampus is sodium dependent and inactivated rapidly at 37 degrees C by a CA++-dependent protease. This recognition site is sensitive to phospholipase A2 and pronase and can be solubilized by Triton X-100. These binding sites are also present in neuroblastoma cell cultures. Many other typical antidepressants but not the atypical antidepressants can displace 3H-imipramine bound to synaptic membranes. Mianserin, an atypical antidepressant, also binds to specific synaptic recognition sites. Destruction of 5HT neurons by intracerebral injection of 5,7 dihydroxytryptamine reduced the number of imipramine binding sites in several brain areas, whereas the number of mianserin binding was increased. Depletion of 5HT by p-chlorophenylalanine increased the mianserin binding sites without modifying characteristics of imipramine binding. Thus some imipramine binding sites are located on serotonergic terminals, while some mianserin binding sites could be associated with post-synpatic serotonergic receptors. Available evidence also suggests that the binding sites of imipramine and those of mianserin are not exclusively associated with only one biochemical type of synapse.